Leishmaniasis, a human disease caused by the parasitic protozoa of the genus Leishmania, is a major scourge in the Tropical World. In contrast to its mammalian hosts, most of the mature mRNAs in Leishmania have a 39 nt signature sequence at their 5'-ends. Without this miniexon sequence, which is transspliced during the maturation of precursor RNAs, the transcripts are not translated into proteins. Phosphorothioate oligodeoxynucleotide (ODN) antisense to this signature sequence of the parasite cells has been shown to be lethal to the parasite proliferation in cultured macrophages, particularly when targeted to the parasitophorous vacuoles by receptor-mediated delivery. The specific aims of this project are to extend this study by (i) optimizing the chemistry of the ODN to effect entry through the parasite plasma membrane, to increase stability and to lessen toxicity to mammalian hosts; (ii) improving the modality of receptor-mediated delivery of this ODN by optimizing the method of conjugation of the ODN with maleylated serum albumin, an artificial but very effective ligand of macrophage scavenger receptors; (iii) evaluating the antileishmanial efficacy of this ODN with respect to its pharmacokinetics and potential toxicity in Leishmania infected mouse model; and (iv) testing the feasibility of the optimized antisense approach in inhibiting the function of specific genes in Leishmania. The long term goal of this study is to develop know-how for research and therapeutic applications of antisense oligonucleotides in the study of leishmanial parasites. Successful completion of this study will not only advance our knowledge towards developing an efficient and specific antileishmanial chemotherapeutic strategy but will also optimize an alternative technology for gene knockout experiments in Leishmania and related organisms.